1. Field of the Invention
The present invention relates to a method for manufacturing a diamond single crystal substrate, and more particularly relates to a method for manufacturing a large-surface area and high-quality diamond single crystal substrate, and to this diamond single crystal substrate, which can be used in semiconductor materials, electronic components, optical components, and so forth.
2. Description of Related Art
Diamond has many outstanding characteristics as a semiconductor material not found in other materials, such as its high thermal conductivity, high electron and hole mobility, high dielectric breakdown field, low dielectric loss, and wide bandgap. In recent years there has been particularly active development of field effect transistors having excellent high-frequency characteristics, and ultraviolet light emitting devices that take advantage of a wide bandgap.
Utilizing diamond as a semiconductor requires a high-quality single crystal substrate, just as with other semiconductor materials. At present, diamond single crystals are mainly obtained industrially by high-temperature, high-pressure synthesis methods, which result in better crystallinity than naturally occurring crystals, and these crystals have physical properties that allow them to be used as semiconductor substrates. However, the size of single crystals that can be obtained with today's high-temperature, high-pressure synthesis methods is limited to the order of one centimeter. The problem with such a small substrate is the semiconductor wafer process, which makes use of electron beam exposure, a stepper, or other such technology that is used in the micro fabrication of silicon and other ordinary semiconductors. A small substrate makes it difficult to use these processing apparatus that were designed with wafers a few inches in diameter in mind, and even if there were a processing apparatus specially intended for small substrates, this would not resolve the difficulties encountered in peripheral steps, such as the photoresist coating step.
One method that has been disclosed for obtaining a diamond single crystal substrate with a large surface area is to prepare a diamond single crystal consisting of low-index planes, and homoepitaxially grow diamond over this by vapor phase synthesis (Patent Document 1: Japanese Patent Publication 11-1392A).
Another method that has been disclosed involves forming a substrate that will serve as the nucleus of vapor phase growth by disposing a plurality of high-pressure phase substances having essentially mutually identical crystal orientations, and growing a single crystal over this by vapor phase synthesis, thereby obtaining an integrated, large single crystal (Patent Document 2: Japanese Patent Publication 3-75298A).
When diamond single crystals are grown by the method in Patent Document 1, the expansion growth rate in the lateral direction will never in principle exceed the growth rate in the upward direction. Obtaining a large-surface area substrate requires corresponding growth in the upward direction as well, which not only is inefficient, but when the crystal is grown rapidly in the upward direction, it is difficult to maintain the single crystal growth conditions in the lateral growth region.
When the method for obtaining a large single crystal described in Patent Document 2 is used, a single crystal seed substrate consisting of a plurality of plates usually does not have exactly the same orientation of the growth planes, and each plate has a slightly different planar orientation. When single crystal vapor phase synthesis is conducted using this substrate and the single crystals are integrated, the joined portions thereof have growth boundaries of different angles, called small angle boundaries, which are defects in the broad sense, and these basically do not disappear no matter how long the single crystal growth is continued. As a result, the semiconductor properties of the regions straddling the small angle boundaries are inferior to those of perfect single crystals, and when a device or the like is produced on the integrated single crystals, performance suffers at the portions that straddle the small angle boundaries.